Not mandatory to donate. Mandatory to make an explicit choice of whether to donate or not (neither opt-out nor opt-in).

I believe GP was suggesting that making a decision should be mandatory at some point. While I personally prefer the default here be to opt in as a donor, GP suggested merely that a decision as to yes or no should be required.

Where I live, one must state organ donor preference when obtaining a driver's license. I believe the default answer is no if you choose not to choose.